1. Field of the Invention
The present invention relates to a solid-state image pickup device used as, for example, a barcode reader (also referred to as a “barcode scanner”).
2. Description of the Related Art
A known type of linear sensor adopted in a barcode reader has, for example, the configuration shown in FIG. 1.
The barcode reader includes a sensor array 10 in which a plurality of photosensors 11 are arranged in a first direction so as to generate signal charges in accordance with the amount of light received, a CCD transfer register 20 disposed along the sensor array 10 so as to read out the signal charge generated by each of the photosensors 11 in a second direction and to transfer the signal charge in the first direction, and a read-out gate (ROG) 30 disposed between the sensor array 10 and the CCD transfer register 20 so as to read out the signal charges from the photosensors 11 toward the CCD transfer register 20.
The signal charge transferred by the CCD transfer register 20 is sent to an output amplifier having a charge-voltage conversion function, is converted into a voltage signal, and is then output.
In the above known barcode reader, however, in response to demands for increased sensor sensitivity, the photosensors 11 extend in the second direction, as shown in FIG. 1, in order to read special markings, that is, barcode labels. For this reason, the chip size is increased, and the cost of the entire solid-state image pickup device is increased.
Furthermore, since the photosensors 11 of the sensor array 10 extend in the second direction, the reading time from the sensor array 10 to the CCD transfer register 20 is longer than in a case using normal photosensors of a square array shape. Moreover, the amplitude of a pulse necessary for reading (a so-called read-out gate pulse φROG) is increased, and a low supply voltage that is required of the barcode reader is impossible.
In addition, since the known barcode reader adopts a reduction optical system because a fixed depth of field is necessary, the image of an object is reduced in size and is focused to the sensor array with regular convex lenses.
In this case, however, a signal difference (called “shading”) is caused between the center portion and the peripheral portion of the object because of the lens characteristics.
While arithmetic processing has hitherto been performed by a subsequent signal processing section in order to correct the shading, when the object is a barcode, the amount of incident light varies depending on the reading conditions, and therefore, the shading cannot be completely corrected, and a reading error arises.
Moreover, since not all of the sensors arranged in the first direction of the sensor array are used, and only the sensors near the center are used, the performance of the pickup device is not fully used, and this is obviously inefficient.
While barcode readers adopt a light source for emitting light with a predetermined wavelength (660 nm in general) depending on the special characteristics of the object, some of them use an optical filter that prevents signal saturation caused when the incident light has another wavelength. This increases the cost of the parts.